Dental furnaces for firing dental restoration parts have been known for a long time. In dental furnaces, a dental restoration part or a number of dental restoration parts are subjected to a firing process or sintering process, in accordance with a precisely prescribed temperature profile.
The quality of the dental restoration parts produced depends essentially on the intended parameters which are optimally adapted for the dental restoration material and are exactly adhered to during the firing process. This comprises not only the temperature profile to be adhered to, not inside the furnace but at the dental restoration part itself, but also the existing vacuum conditions during the firing cycle.
Such dental restoration parts comprise dental restoration parts to be manufactured of plastics, of metal, of composites, and, in particular, also of ceramics.
Ceramic dental restoration parts are often produced in what is referred to as muffles, in which hollow molds correspond to the precise shape of the future dental restoration part as negative impressions. Such patterns are made of wax or similar materials as positive patterns, are mounted on a common basis via what is referred to as waxing sprues, and are then cast out with a material which cures true to shape and which in most cases contains plaster.
After the wax has been melted out, hollow spaces are available, with the waxing sprues being connected with one another via a common press channel which in most cases is of cylindrical shape, and ending at the hollow spaces in which the dental restoration parts are to be produced.
When a dental restoration part is to be pressed, a green body or press blank is introduced into the press channel and is subjected to pressure with the help of a molding plug. During the process of heating to reach the sintering temperature of the ceramic material of which the press blank consists, the press blank begins to take a plastic state and is introduced into the hollow shaping spaces for the production of dental restoration parts by means of pressure exerted on the molding plug.
This process has been known for a long time, but lately attempts have been made to improve the throughput of dental press furnaces, for instance by pressing more or larger dental restoration parts together in one muffle.
Larger hollow shaping spaces or a larger number of shaping spaces, however, lead to a certain weakening of the muffle. For this reason, the furnace parameters for preventing the formation of cracks in the muffle or other damage of the dental restoration parts must be adhered to even more precisely.
It has already been suggested as well, as can be seen from DE 10 2006 050 830 A1 and corresponding US Published Application No. 2008099939, which is hereby incorporated by reference, to monitor whether the muffle which is subject to pressure exhibits a crack during the pressing process. This evaluation, however, can basically be done after the fact, and it would be desirable to safely prevent cracks in the muffle already in advance.
In order to guarantee this with a certain safety margin, the muffles produced are subjected to a prescribed press force to be set by the user, which force is clearly lower than the press force at which cracks might appear.
A reduced press force, on the other hand, offers the risk of the dental restoration parts having to remain in the firing furnace for too long, so that the hollow shaping spaces are completely filled, and also the risk of the user by accident interrupting the pressing process because the user assumes that the pressing process has been completed, for instance when the molding plug does not move further downwards. This can happen, in particular, when there is comparably high friction inside the shank. A dental furnace operated at reduced press force is more vulnerable for such friction faults in comparison to a dental press furnace operated at high press force.
Another quality problem is that parameters are by accident set wrongly for the firing process and/or the pressing process to be carried out. This can result in the press force being set too high or too low, or in the firing temperature being set too high or too low. The dental restoration part produced will then be of minor quality, which in many cases will go unnoticed because it will not recognizable in the finished dental restoration part. Damage will in this case occur only after the part has been used inside the mouth of the patient, in different cases often for years, for which then the dentist or the producer of the dental restoration material will be blamed, even if it was indeed the dental technician operating the press furnace who made an error.
In large dental laboratories, the work process is often split, such that certain dental technicians do only certain jobs with given parameters, which to put into practice will quite possibly also be feasible at the same firing furnaces which in most cases cover a rather large range in accordance with the client's wishes. In order to limit the range of parameters for the necessary dental technical work, it has already been suggested to draw up what is referred to as user profiles which make it necessary for the respective user to register with the dental furnace before firing curves and press curves available for selection within his range are presented to him. For a correct assignment, however, it is vital that the user also logs off when he has finished his job at the firing furnace concerned, which is thought to be awkward, such that it is to be feared that another user will accidentally carry out some wrong operation, assuming that the dental furnace concerned is presenting him the “correct” firing curves and press curves for selection.